Among standard color spaces for computer display, there is the sRGB standard prescribed by IEC (International Electro-technical Commission). This standard gives a definition on the relationship between a video signal RGB and the calorimetric values by having chromaticity points of three prime colors of red (R), green (G) and blue (B) coincide with the colorimetric parameters of Rec.709 recommended by ITU-R (International Telecommunication Union Radio communication). In a display apparatus, complying with this sRGB standard, if a video signal RGB is applied, the colorimetrically same color may be displayed.
Meanwhile, with a picture unit, receiving and displaying the color information, captured by a camera or a scanner, such as a display or a printer, it is essential to demonstrate the received color information accurately. For example, if a camera has captured the color information accurately, but a display demonstrates the color information only inappropriately, color reproduction performance of the system on the whole is deteriorated.
In a current standard monitor device, the display is prescribed by the color gamut of the sRGB standard. In actuality, there are many colors beyond the color gamut of sRGB, such that there are object colors that cannot be represented by a standard monitor device complying with the sRGB standard. For example, with a halide film used in a camera, or with a digital camera printer, the range of sRGB has already been exceeded. If the broad dynamic range is procured, and an image pickup operation is carried out correctly, there are produced object colors that cannot be represented on a standard monitor device of the sRGB standard.
The sYCC, having a color space broader than that of sRGB, has been adopted as a standard by business circles, in order to cope with the color gamut which has become broader. The sYCC has derived, from the sRGB, the luminance difference color difference separation space, using ITU-R BT.601,which is the international standard of a transformation matrix from RGB to YCC as defined for high vision television. The color gamut of sYCC is broader as the color space, such that, with the sYCC, the color outside sRGB can be represented.
On the other hand, in the NTSC system, adopted as the broadcast system for color television, the bandwidth is broader than in sRGB. If sYCC is to be implemented, the color gamut on the display with sYCC needs to be equivalent to or even exceed that of the NTSC system on a display.
On the other hand, a TV receiver of an extremely thin thickness, such as a liquid crystal display (LCD) or a plasma display panel (PDP), has been developed and put to practical use, to take the place of the cathode ray tube (CRT) which has long been used since the start of TV broadcasting. In particular, a color liquid crystal display, employing a color liquid crystal display panel, is expected to become popular at a rapid rate because it permits driving with low power consumption and the large-sized color liquid crystal display panel has become less expensive.
As for the color liquid crystal display apparatus, the backlight system, in which a transmissive color liquid crystal display panel is illuminated from its backside with a backlight device to display a color picture, is in the mainstream. The light source, preferentially used for the backlight device, is a CCFL (Cold Cathode Fluorescent Lamp), emitting white light using a fluorescent tube.
In general, in a transmissive color liquid crystal display apparatus, a color filter, employing a tristimulus filter of spectral characteristics, shown for example in FIG. 1, made up of a blue filter CFB0(460 nm), a green filter CFG0(530 nm) and a red filter CFR0(685 nm), where the numbers entered in parentheses denote the peak transmission wavelength of each filter, is provided from one pixel of the color liquid crystal display panel to another.
On the other hand, the white light, emitted from a three-wavelength CCFL, used as a light source for a backlight device of the color liquid crystal display apparatus, has a spectrum shown in FIG. 2, such that it contains light of different intensities in a variety of wavelengths.
Hence, there is a problem that the color reproduced by the combination of the backlight device, having such CCFL, emitting the light of three wavelength ranges, as light source, and the color liquid crystal display panel, having the color filter, described above, is rather poor in color purity.
FIG. 3 shows the color reproducing range of the color liquid crystal display apparatus, including the backlight device, having the above-described three-wavelength CCFL as a light source. Specifically, FIG. 3 depicts an xy chromaticity diagram of the XYZ color system, as prescribed by the Commission Internationale de l'Eclairage (CIE).
As may be seen from FIG. 3, the color reproducing range of the color liquid crystal display apparatus, having the backlight device, employing the CCFL as light source, is narrower than the color reproducing range provided for by the standard of the NTSC (National Television System Committee) system adopted as the color television broadcasting system. That is, the former color reproducing range may not be said to cope sufficiently with the current television broadcasting.
On the other hand, there is a fear that the CCFL, containing mercury in the phosphorescent tube, may have an ill effect on the environment. Hence, a demand is raised for a light source that may take the place of the CCFL as a light source of the backlight device. With the development of the blue light emitting diode, the light emitting diodes, emitting light of three prime colors, namely red light, green light and blue light, are now in order. Thus, with the use of the light emitting diodes as light source for the backlight device, the color light obtained by the color liquid crystal display panel may be improved in color purity, and hence it may be expected that the color reproducing range may be made as broad as or even broader than the color reproducing range provided for by the NTSC system.